This invention relates to tuning fork-type piezoelectric vibrators operated with suppressed vibration modes of higher orders which are undesirable in operation of such piezoelectric vibrators.
In a tuning fork-type piezoelectric vibrators of piezoelectric material such as single crystal piezoelectric body of quartz, lithium tantalate, lithium niobate or polycrystalline piezoelectric ceramics, various electrode arrangements have heretofore tried so that said piezoelectric vibrators may have desired flexural vibration modes according to the cutting angles of the crystals used.
In FIGS. 1a and 1b which are shown respectively in a tuning fork-type piezoelectric vibrator body, hatched portions indicate electrodes for effecting flexural vibration in the tuning fork-type piezoelectric vibrator bodies. It is seen particularly from FIG. 2a that split electrodes 3, 4, 5, 6, 7, 8, 9 and 10 are provided on the faces of legs 111 and 112 extending from base portion 201 of the vibrator body. Said faces of the legs are contained respectively in the principal planes or planes B.sub.1 C.sub.1 shown in FIG. 1a. And, it is seen particularly, from FIG. 2b that so-called four side electrodes 10, 11, 12, 13, 14, 15, 16, and 17 are provided on the faces of legs 121 and 122 extending from base portion 202 of the vibrator body. Said faces of the legs are contained in the principal plane B.sub.2 C.sub.2 or the side plane A.sub.2 C.sub.2. As apparent from FIGS. 1a and 1b, a tuning fork-type piezoelectric vibrator is formed in a U-shape having principal faces contained in plane B.sub.1 C.sub.1 or plane B.sub.2 C.sub.2.
The crystal axes A.sub.1, B.sub.1, C.sub.1 and A.sub.2, B.sub.2, C.sub.2 are respectively cubic rectangular coordinate axes. And, the direction of cutting of a tuning fork-type piezoelectric vibrator using coordinate axes commonly used of crystal is expressed as follows: For quartz crystals, A.sub.1 is X, B.sub.1 is Z' and C.sub.1 is Y'; and A.sub.2 is Z', B.sub.2 is X and C.sub.2 is Y'. For lithium tantalate crystals, A.sub.1 is Z', B.sub.1 is X' and C.sub.1 is Y"; and A.sub.2 is X", B.sub.2 is Z' and C.sub.2 is Y". And, for lithium niobate crystals, because it has no zero-temperature coefficient, the directions of cutting with respect to FIG. 1a and FIG. 1b can be expressed such that A.sub.1 is an axis other than the X-axis and C.sub.1 is an axis other than the Z-axis, and B.sub.2 is an axis other than the X-axis and C.sub. 2 is an axis other than the Z-axis respectively.
Hereon, the coordinate XY'Z' indicate a coordinate which is the original coordinate XYZ pivoted around the X-axis. The coordinate X'Y"Z' indicates a coordinate which is the original coordinate XYZ pivoted around the Z-axis, to cause the coordinate X'Y'Z and subsequently pivoted around the X'-axis.
To pivot a coordinate such as stated above is to pivot the direction of cutting of a piezoelectric body at a desired angle so as to obtain a frequency characteristic suitable for the temperatures to be used. This is normal means in a design of piezoelectric flexural vibrators. And, for a polycrystalline piezoelectric ceramic body, the plane of flexure is to be polarized since it does not have the axes of crystal such as in a piezoelectric body of single crystal.
Referring now to FIGS. 2a and 2b, tuning fork-type piezoelectric vibrators such as shown in FIGS. 1a and 1b will vibrate respectively in the B.sub.1 C.sub.1 -plane and the B.sub.2 C.sub.2 -plane for the flexural vibration upon impression or between terminals D.sub.1 and E.sub.1 and terminals D.sub.2 and E.sub.2, of an alternating voltage corresponding to the resonant frequencies of said tuning fork-type piezoelectric vibrators. A tuning fork-type vibrator as shown in FIGS. 1a and 1b can be secured firmly by supporting the same at the principal faces or the side faces of the base portion 201 or the base portion 202 of the respective vibrator body.
Now, upon supporting the vibrator body at any portion of said body, a tuning fork-type piezoelectric vibrator produces usually vibration modes of higher orders other than the desired vibration mode of fundamental frequency. Moreover, particularly in such a small-sized tuning fork-type vibrator for being incorporated in a wrist watch, vibration modes of higher orders are produced easily due to the structure of electrodes provided on the faces of such a vibrator body. In such a vibrator body, the electrodes as indicated at hatched portions in FIGS. 1a and 1b are formed as large as possible so as to prevent the series resonant impedance of the vibrator from being increased. Such a structure of the electrodes in a relatively small tuning fork-type vibrator causes the series resonant impedance for the vibration modes of higher orders to be smaller than that for the vibration mode of fundamental frequency.